printk

Printk is the primary logging facility used by the Linux kernel to emit messages from kernel code. It formats messages and writes them to the kernel log buffer, and, depending on console configuration, to the system console as well. Unlike user-space I/O, printk runs in kernel space and uses the kernel’s own memory and synchronization mechanisms.

Usage of printk centers on log levels and formatting. Messages are produced by calling printk with an optional log level prefix, such as KERN_INFO or KERN_ERR, for example: printk(KERN_INFO "Device ready: %s\n", devname); In modern kernels, common wrappers like pr_info, pr_err, and pr_debug are used, which call printk with fixed log levels. The log levels range from emerg, alert, crit, err, warning, notice, info, to debug.

Logging storage and viewing: printk messages are stored in the kernel log buffer and can be retrieved with tools like dmesg or read via /proc/kmsg, depending on configuration. The buffer is finite and subject to overflow; when full, older messages may be dropped or overwritten. The amount and timing of visible messages depend on the configured loglevel and the active console(s).

Boot and runtime considerations: Early printk allows a minimal set of messages before the system console is available during boot. The printk subsystem supports rate limiting and dynamic debugging to reduce overhead in production. Output is directed through the kernel console subsystem, which may be serial, VGA text, or other hardware, depending on configuration.

Printk remains a fundamental tool for kernel developers and for diagnosing issues, providing a standardized way to report status, errors, and debug information from kernel code.